1. Field of the Invention
The present invention relates to a method of driving a plasma display panel (PDP) of AC type and a display apparatus using the method.
With the practical application of the color screen as a motive, PDP has come to be widely used for various applications including the TV picture and the computer monitor. It has also been closely watched as a means for realizing a large screen of high-definition TV. For developing the high definition and large screen of PDP further, it is necessary to reduce power consumption while securing the display quality.
2. Description of the Related Art
The PDP of AC type has such a structure that in order to sustain the on-state utilizing the wall charge, the main electrodes are covered with a dielectric material. In display, only the cells to be turned on (to emit light) are charged by the line-by-line addressing operation, after which all the cells are supplied with the sustain voltage Vs of alternate polarities at the same time. The sustain voltage Vs satisfies equation (1). EQU Vf-Vwall&lt;Vs&lt;Vf (1)
where Vf is the discharge start voltage, and Vwall is the wall voltage.
In cells having the wall charge, the wall voltage Vwall is superimposed on the sustain voltage Vs, and therefore the effective voltage (also called the cell voltage) Veff applied to the cells exceeds the discharge start voltage Vf and causes a discharge. In the case where the sustain voltage Vs is applied in a shorter period of time, the apparently continuous on-state is obtained. The brightness of display is dependent on the number of times the discharge occurs per unit time. The halftone, therefore, is reproduced by appropriately setting the number of times the discharge occurs in each field (in each frame for no-interlace system) for each cell according to the gradation level. The color display is a kind of gradation display and the display color is determined by a combination of the brightness of the three primary colors.
In a widely-known method of displaying the gradation with PDP, each field is configured of a plurality of subfields weighted by brightness, and the total number of times the discharge occurs per field is set by a combination of subfields turned on and off (as disclosed in JP-A-04-195188). The "weight of brightness" or "brightness weight" is a numerical value (normally given by an integer with a minimum value of unity) for determining which subfield(s) is selected for turning on according to the gradation of the input image. Generally, what is called the "binary weighting" is employed, in which the weight of each subfield is expressed as 2.sup.n (n=0, 1, 2, 3 . . . ). Assuming that there are eight subfields, for example, 256 gradations from 0 to 255 can be displayed.
The binary weighting has no redundancy and is suitable for multiple gradations. For securing a uniform gradation width (the brightness difference for one gradation step) over the whole gradation range, however, the addressing operation is required for each subfield. Also, in at least one subfield of each field, a resetting process (address preparation) is required to uniform charged state in the whole screen before addressing. If the resetting process is omitted, the discharge condition varies between the cells remaining with the wall charge (previously turned-on cells) and the other cells (previously turned-off cells), thereby making difficult accurate addressing operation. Normally, the resetting process is performed in each subfield for improving the reliability of the addressing operation.
Since the resetting and the addressing operations are accompanied by the discharge, however, these processes are desirably performed as few times as possible from the viewpoint of contrast and power consumption. Especially for the high-definition PDP in which a large burden is imposed on the circuit parts for the addressing operation, a reduced number of times of addressing is desired if only to suppress the heat generation.
A driving method has been proposed in which a predetermined number of subfields are separated into a plurality of subfield groups in each of which the resetting process is performed once (Japanese Patent No.2639311). The subfields of each subfield group are equalized in weight, and the weight of each subfield is determined by adding a minimum weight to the total of weights smaller than the weight of the particular subfield. In this way, the gradation width can be equalized over the entire gradation range.
In the conventional method, the number of times the sustaining discharge occurs (i.e. the number of times the sustaining voltage is applied) is uniquely set for a given weight of brightness, and therefore the number of times of sustaining discharge is the same in each subfield whose weight is equal to one another's.
In the gradation display described above in which each field is configured of a plurality of subfields, a combination of subfields in which the total weight assumes a value corresponding to the required gradation is selected to be turned on, and the total weight of the selected subfields is proportional to the gradation of the input image.
Although the larger the number of times the sustaining discharge occurs, the higher the actual brightness, the two are not proportional to each other. In other words, the brightness tends to be saturated as the number of times of the sustaining discharge increases. This poses the problem that the reproducibility of the bright side of the gradation range is lower than that of the dark side thereof.